Radio receiver



April 20, 1937. CIJ, FRANKS 2,078,072

RADIO RECEIVER Filed sept. 9, 193s 2 sheets-sheet i itl- /aoa Eefde/Icy /ud/'o F)- fil April 20; 1937. c. J. FRANKs 2,078,072

RADIO RECEIVER Filed Sept. 9, 1953 y '2 Sheets-Sheet? Patented Apr. 2i), 1937 Ui'i` STARS ATENT RADIO RECEIVER Delaware Application September 9, 1933, Serial No. 688,846

10 Claims. Cl. Z50-20) l This invention relates to radio receivers and particularly to a receiver of the automatic gain control type. The invention is not limited to, but is particularly useful in connection v/ith, receivers of the so-called midget type in which it is desirable to reduce the size of the receiver and the number of tubes.

Attempts to reduce the number of tubes in a receiver have been accompanied by a loss in sensitivity or a lack of good tone quality. Unfortunately, the expediente employed to avoid one of these objections have usually rendered the other feature still more objectionable. An object of the invention is to provide circuit arrangements which insure both relatively high amplification and freedom from distortion. An object is to provide a radio receiver of the automatic gain control type, and in which at least one controlled tube acts as both a radio frequency and audio frequency amplifier.

A further object is to provide a receiver in which a single. tube operates as a radio frequency amplier, a detector, an audio frequency amplifier and the source of an automatic gain control voltage. More particularly, an object is to provide a receiver in which a single tube which acts as a radio frequency amplier and detector, has both the audio frequency and direct current components of the rectied voltage fed back thereto, whereby the gain of the radio frequency amplifier is automatically controlled and the tube performs the additional function of an audio amplifier.

These and other objects and advantages of the invention will be apparent from the following specification when taken with the accompanying drawings in which:

Fig. l is a schematic diagram illustrating the general method of operation in accordance with the invention;

Fig. 2 is a circuit diagram of an embodiment of the invention;

Fig. 3 is a fragmentary circuit diagram illustrating a variation of the control system of Fig. 2; and

Figs. 4 and 5 are circuit diagrams of other embodiments of the invention.

In the drawings, the reference numeral l identifies a radio frequency amplifier which is followed i by a second amplifier 2 that works into a combined demodulator and rectifier t, the path of the radio frequency current being indicated by the plain arrows a. As shown by the single feathered arrows b, the audio frequency output of the demodulator 3 is returned to the amplier 2, and

the amplified audio output may be amplied further by the tube amplifier l or fed directly to the loud speaker 5 if the audio power available from the reiiexed amplifier is sufcient to produce the desired sound output. The direct current output of the demodulator-rectifier 3 is returned to the radio frequency amplifier l and to the reilexed amplifier 2 as a gain control voltage.

Although a plurality of cascaded stages may be included in one or more of the amplifiers, the invention is of particular advantage when a minimum number of tubes are employed. The reflexing of one ampler stage has the obvious advantage of increasing the sensitivity of the receiver. The increased sensitivity does not, however, result in poor quality of reproduction since the automatic control of the amplication of the reflexed stage operates, in conjunction with the gain control of the preceding stage or stages, to hold the input voltages upon the detector and audio ampliers within the relatively narrow ranges which will not result in distortion.

As shown in Fig. 2, the first radio amplifier includes a multiple element tube 6 which functions both as a carrier wave amplifier and a selfoscillating detector. The input system for tube 6 preferably includes two tuned circuits i which are loosely coupled, to obtain high selectivity before impressing the carrier wave voltage upon the control grid G3. The grid coil 8 of the oscillator system is connected between grid G1 and ground through the usual grid leak arrangement comprising condenser 9 and resistance 9', and the coupled anode coil il is connected between grid G2 and a source of positive potential, indicated as -l-D. The screen grid G4 may be connected to the same source -l-D through a resistance lil. The tuning condenser Il of the oscillator network is preferably connected mechanically to the tuning condensers of circuits l, and series condenser 9 is adjusted to provide a proper tracking to permit single control tuning for the receiver.

The intermediate frequency developed in the plate circuit of tube S is transferred by the usual transformer coupling l2 to the grid G1 of tube I3 which preferably takes the form of a pentode having an additional anode A. The plate circuit of tube I3 includes the primary of an intermediate frequency transformer ifi, 'the secondary being connected to anode A and, through resistance l5, to the cathode K of the tube. The resistance I5 is shunted by an intermediate frequency bypass condenser I6, and it will be apparent that both audio frequency and direct current voltages will be developed across the resistance I5 by the diode rectifier action of elements A and K.

The audio and direct current voltages are returned to the input circuit of the tube I3 through the low pass filter, formed by resistance Il and condensers I8, which passes audio frequency and direct currents but not intermediate frequency currents. The plate circuit of tube I3 includes the audio frequency inductance or choke coil I9 that may be shunted by a resistance 2U to provide a desired frequency-impedance characterstic. A voltage divider 2l is connected through a blocking condenser 22 between ground and the junction of the intermediate frequency and audio frequency impedances in the plate circuit of tube I3. The adjustable slider ofthe voltage divider provides a manual control of the output level at Vreproducer 5 by a suitable adjustment of ther audio voltage impressed upon the control grid of the audio amplifier tube 23.

Reverting to the diode rectifier, the direct current output developed across resistance I5 is impressed upon tube 5 through lead 24 and the audio frequency filter formed by resistance 25 and the condenser 26. The translation gain of the oscillating detector 5 is thereby adjusted automatically to reduce the variations, with changing signal strength, of the radio frequency input to the refleXed amplifier tube I3.

In the absence of a further control of the ampliiier gain, strong signals would be so highly amplified that the audio amplifier tube 23 and possibly the reflexed amplifier tube I3 would be overloaded, thus resulting in distortion. By controlling the gain of the refiexed amplifier, the audio voltage developed by tube I3 may be held at 'a relatively high level without danger of overloading.

It is to be noted that the reiiexed amplifier tube I3v should be of the type having a multi-mu characteristic to prevent too rapid a decrease in the audio frequency gain as the strength of received signals increases. Although it is not practical, in general, to control audio frequency gain solely by varying a direct current bias on an amplifier tube, the automatic control of the radio. frequency amplifier reduces the range of control which need be applied to, the tube I3, and

therefore eliminates the distortion whichY would result if itv were attemptedjto control the audio output level of tube I3 solely by varying thev audio frequency gain.

While a primary purpose of the invention is to prevent an overload at any point in a radio receiver, as should be the function of all automatic gain control systems, the invention presents the further advantage that the audio level may be held substantially constant over a wide range of receiver input voltages, Since the same radiov voltage is impressed upon the detector and the rectifier (or combined detector-rectifier) in the usual gain control system, it is obvious that there will be an appreciable variation in the output level as thel receiver input varies over a wide range. According to the present invention, the rise in detector input for increasing signalI strength may be substantially neutralized by the automatic reduction in the audio gain beyond the detector.

As yshown in Fig. 3, it is possible to apply any desired fraction of the audio output of the diode rectier to the input circuit of tube I3,V and to apply the same or any other desired fraction of the available direct current voltage to the radio amplier 6 and to the refiexed tube I3. As

the radio amplifier may be of the type shown in Fig. 2 or of any desired type, the fragmentary view does not illustrate any of the elements of Fig. 2 which precede the transformer I2, and such illustrated elements as are or may be identical with those of Fig. 2 are identified by corresponding numerals but will not be described in detail.

The audio frequency circuit between the diode leak I5 and the input circuit of tube I3 comprises a tap 21 which is adjustable along resistance I5 and a condenser 28 having a low impedance for audio frequency currents. As the tap 2l provides a control of the output level, the voltage divider 2I of the Fig. 2 circuit may be replaced by a simple resistance 2|' which completes the direct current circuit for biasing the control grid of tube 23.

A desired fraction of the available control voltage is selected by adjustment of the tap 29 and returned to the radio amplifier through the filter network I'I, I8 and the lead 2li. A similar tap 30 provides means for determining that fraction of the available control voltage which is returned to the' control grid of tube i3 through the filter network I'I, I8.

Another method of applying different control voltages to the controlled tubes is illustrated in Fig. 4l. The radio amplifier includes one or more tubes 6 for amplifying at the received carrier frequency and/or at an intermediate frequency and a reflexed tube I3' which includes anodes A, A in addition to the amplifier electrodes. It will be noted that the cathode return of tube or tubes 6 is not made to ground or the negative terminal of the plate current source SI, but the cathode circuit which includes the biasing resistor 3| is returned to a point X of positive potential. So far as concerns the development of audio frequency and gain control voltages for application to the control grid of tube I3', the circuit arrangement may be substantially the same as that illustrated in Fig. 2, and the various elements are identified by the correspondingreference numerals of that view.

The second anode A is connected to the positive potential point X through a resistance 32, and a resistance 33 is connected between the cathode and the negative terminal of the plate current source 3I. The control bias lead 24 of the tube or tubes 6 is connected to the anode terminal of resistance 32 through the filter formed by resistance 34, and condenser 35. It will be noted that the voltage divider 2I for feeding the succeeding stage or reproducer is connected to the cathode through the blocking condenser 22.

The tube I3 functions as a radio amplifier, a rectifier for developing audio and'V direct current voltages which are applied to the control grid', and as an audio and direct current amplifier for developing amplified voltages across the cathode resistance 33.

In the absence of a. received signal, the relatively heavy fiow of plate current develops a voltage across resistance 33 which places the cathode at a high positive potential. This potential is applied to the control grids of tube or tubes 6' and is such that the bias between such grids and the positively polarized cathodes is appropriate for high gain. The direct current flow through resistance 33 Varies Vwith received signal strength sinceA the direct current output` across resistance I5 is impressed upon the control grid G1 of tube I3', and in such sense that the bias on this grid becomes morewnegative as thesignal 'Ihisautomatic change in the bias on grid Gi reduces the stage gain at both radio and audio frequency and also reduces the flow of plate current, thus lowering the positive potential of the cathode end of resistance 33. The direct current voltage variations across resistance 33 are amplified reproductions 0f the voltage changes across the diode resistance l5, and current iiows between elements A', K when the cathode potential falls below the potential established on anode A from point X. The current flow thus established causes a potential drop across resistance 32 which makes the grids 0f tube or tubes 6 more negative, thereby controlling the gain of the radio amplifier,

This general method of and circuit arrangements for developing an amplified gain control voltage is described and claimed in the copending application of Paul O. Farnham, Serial No. 655,117, filed Feb. 3, 1933.

The described circuits effect a reflexing of only the final radio amplifier tube but it will be apparent that any or all of the tubes may be employed for both radio and audio frequency amplification in accordance with the known reflexing methods.

As illustrated in Fig. 5, the reiiexing may be applied to one or more of the tubes 6a preceding the nal radio amplifier tube I3. The tube i3 may be of the type shown in Fig. 2, and forms part of a combined radio amplifier-detector-gain control system which develops both an audio frequency and a direct current voltage across the diode resistance l5. Only the direct current voltage, or a desired fraction thereof, is impressed upon the control grid of tube I3 through the filter comprising resistance ld and a condenser lila which has a low impedance for audio frequencies. Both the audio and direct current voltages are impressed upon one of the radio amplifier tubes 6a through lead 24 and a filter comprising resistance il' and condensers i3 which have a low impedance for radio frequencies. When additional audio amplification is not required, the loud speaker 5 may be included in the output circuit of a reflexed tube. For simplicity, only one tube 6a is shown in Fig. 5, but it is to be understood that there may be a plurality of tubes 6a and that any number of such tubes may be reiiexed, and that tube I3 may also be included in the reflex circuits. No means for controlling the audio output level is illustrated as the manual level control may be included in the antenna input system, may be an adjustment of a biasing potential on one of the radio amplifiers or may be a control of the audio voltage returned to a refiexed stage, as shown in Fig. 3.

It will be apparent that the invention is not limited to the particular circuits herein illustrated and described, and that many variations may be made without departure from the spirit of my invention as set forth in the following claims.

I claim:

l. In a radio receiver, a detector, a reflexed amplier stage comprising an amplifier tube havstrength increases.

f ing input and output circuits including radio frequency impedances, means coupling the detector to the impedance in the output circuit, circuit elements connecting the detector output circuit to said amplifier to impress the audio output of said detector upon said amplier tube, and an audio frequency impedance in the output circuit of said amplifier tube; and means for automatically controlling the gain of said amplifier tube in accordance with variations in the strength of received radio signals, wherein the said means for controlling the amplifier gain comprises the said circuit elements for returning the audio output of said detector to said amplifier tube.

2. In a radio receiver of the superheterodyne type including a multiple electrode tube functioning as an intermediate frequency amplifier and a second detector, said tube including at least a cathode, signal grid and plate providing an amplifier section, an auxiliary anode in said tube providing a diode rectier section with said cathode, an intermediate frequency input circuit connected between the grid an-d cathode, an intermediate frequency output circuit connected to the plate, means impressing intermediate energy in said output circuit upon said diode, a resistor connected with the diode across which audio and direct current voltages are developed, means impressing at least a portion of said audioI voltage upon the said signal grid, an audio output impedance connected to said plate across which appears said last audio voltage in amplified form, an audio utilization network coupled to said impedance, and means impressing at least a portion of said direct current voltage upon said signal grid in a sense to vary the gain of the amplifier section inversely with signal carrier amplitude.

3. In a radio receiver, the combination of a multiple stage radio amplier working into a rectiiier, a rectifier output circuit across which both direct current and audio frequency voltages are f developed, and circuit connections for returning desired fractions of said voltages to at least one of said radio amplifier stages, whereby said one stage functions both as a radio and as an audio amplier and the gain of that stage is automatically controlled in accordance with the direct current output of said rectifier, wherein said Y circuit connections return a desired fraction of only the direct current voltage to one radio stage to control the gain thereof, and return desired fractions of both voltages to another of said radio stages.

4. In a radio receiver, an amplifier tube having an input circuit comprising a radio frequency impedance connected to the contro-l grid of the tube and through a radio frequency condenser to the tube cathode, means for impressing radio voltages upon said impedance, an output circuit for said tube including a radio frequency impedance and an audio frequency impedance, a diode rectifier having an input circuit coupled to the radioI frequency impedance of said output circuit, an output circuit for said rectifier including a resistor across which both audio frequency and direct current voltages are developed, and circuit .connections to said resistor for returning audio frequency and direct current voltages of said rectifier resistor to the control grid of said amplifier tube; whereby said tube functions both as a radio and audio amplifier, and the gain of the tube is automatically controlled by the direct current voltage returned from said rectifier.

5. In a radio receiver, an amplifier tube having an input circuit comprising a radio frequency impedance connected to the control grid of the tube and through a radio frequency condenser to the tube cathode, means for impressing radio voltages upon said impedance, an output circuit for said tube including a radio frequency impedance and an audio frequency impedance, a diode rectifier having an input circuit coupled to the radio frequency impedance of said output circuit, an output circuit for said rectifier and across which both audio frequency and direct current voltages are developed, and circuit connections for returning both the audio frequency and direct current output of said rectifier to the control grid of said amplifier tube; whereby said tube functions both as a radio and audio amplifier, and the gain of the tube is automaticaily controlled by the direct current voltage returned from said rectifier, said means for impressing radio voltages upon the radio frequency impedance ofthe input circuit of said tube comprises a radio frequency amplifier, in combination with circuit elements for returning the direct current output of said rectifier to said radio frequency amplifier thereby to effect an automatic control of the `gain thereof.

6. A radio receiver as claimed in claim 4, wherein the elements'of said diode rectifier are located within said amplifier tube.

7. In ay radio receiver, a radio amplifier including a plurality of cascaded stages, a rectifier Working out of the last of said radio stages to develop both audio frequency and direct current voltages, means for impressing a desired fraction of said direct current voltage upon one of said stagesas a gain control voltage, and means including a direct current amplifier for impressing upon another stage a gain control voltage derived from and of greater magnitude than the direct current vo-ltage developed by said rectifier.

8. A radio receiver as claimed in claim "I, in combination with means for returning the audio frequency output voltage of said rectifier to one of said radio stages, and an impedance in the output circuit of the last-mentioned radio stage across which an amplified audio voltage is developed.

v 9. In a radio receiver, a combined ampiifiergain control stage including a tube housing a cathode cooperating with a control grid and anode to function as an amplifier and with an auxiliary anode to function as a diode, a radio input circuit between said control grid and cathode, an

output circuit between saidcathode and first anode, said output circuit including an impedance across which an amplified radio voltage is developed, a diode rectifier circuit between said cathode and ainriliary anode, said diode circuit being coupled to the said output circuit impedance and including a resistance across which direct current and audio frequency voltages are developed, a conductive connection between said resistance and said control grid for impressing both of said voltages on the amplifier elements of said tube, a resistance in said output circuit across which amplified audio frequency and direct .current voltages are developed, an audio network coupled to the last resistance to utilize said amplified audio voltage, and an amplifier, preceding said tube, having an electrode thereof coupled to the resistance to utilize said amplified direct current voltage.

l0. A radio receiver comprising a multiple element tube having a carrier frequency input circuit connected between the control grid and cathode, an intermediate frequency output circuit for said tube, an oscillator network connected with certain elements of said tube to effect operation thereof as an oscillating detector, an amplifier tube having an input circuit coupled to the output circuit 0i said first tube, a detector upon which said amplifier tube impresses an amplified intermediate frequency voltage, a detector output circuit connected to the input circuit of said amplifier tube to impress thereon the audio and direct current voltages developed by said detector, an audio frequency impedance in the output circuit of said amplifier tube, and an audio frequency amplifier coupled to the output circuit of said amplifier tube, circuit elements for returning the direct current output of said detector to the control grid of said multiple element tube, thereby automatically to control the translation gain of said oscillating detector tube.

CHRISTOPHER J. FRANKS. 

